JP2000347149A - Directional coupler - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a novel directional coupler which is low in drive voltage and is capable of achieving a bar state and cross state only by the control of impressed voltage. SOLUTION: This directional coupler 1 has a substrate 2 having an electro- optic effect, optical waveguides 3-1 and 3-2 of a directional coupler type formed in the substrate 2 and electrodes 4-1 and 4-2. The substrate 2 is divided to a first polarization inversion domain having a polarization direction P approximately perpendicular to the main surface 2A of the substrate 2 and a second polarization inversion domain having a polarization direction Q opposite to the polarization direction P. The electrodes 4-1 and 4-2 are bisected or multi- divided by the boundary surface 7 of the first polarization inversion domain 5-1 and the second polarization inversion domain 5-2.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a directional coupler, and more particularly, to a directional coupler that can be suitably used for an ultra-high-speed switch in an ultra-high-speed communication field.

[0002]

2. Description of the Related Art With the recent development of multimedia, the amount of information has been increasing. For this reason, in the field of ultra-high-speed communication such as optical communication, to control such information,
Transducers having gigabit / sec class processing power are required. Such an ultra-high-speed converter, that is, an ultra-high-speed switch, has been difficult to perform with a conventional electronic switch. For this reason, there is an urgent need to develop an optical switch capable of ultra-high speed switching. At present, a directional coupler type optical switch is generally used as such an ultra-high-speed optical switch. Typical directional couplers include a uniform Δβ directional coupler and an inverted Δβ directional coupler.

In a uniform Δβ directional coupler, two optical waveguides are arranged close to each other by a predetermined length on a substrate having an electro-optical effect, and a uniform electrode is arranged on each of these optical waveguides. It is. Then, voltages having different signs are applied to the respective electrodes to modulate the phases of the light waves guided in the respective optical waveguides, and a high-speed switch is performed by adjusting the coupling ratio of these light waves. In this directional coupler, in order to turn on the cross-state, it is necessary to set the coupling length Lo of each optical waveguide to π / 2κ (κ is a coupling coefficient), which is difficult in manufacturing.

On the other hand, the inverted Δβ directional coupler has two optical waveguides arranged in the same manner as described above, and then two electrodes are juxtaposed on each of these optical waveguides. It has a form divided into. Then, by applying voltages having different signs between adjacent electrodes on the same optical waveguide and between electrodes formed on two different optical waveguides and facing each other, the phase of the guided light is modulated to couple the light waves. A high-speed switch is performed by adjusting the ratio. In this directional coupler, even when the coupling length of the optical waveguide does not satisfy the above relationship, the bar-state is changed from ON to cross by controlling the applied voltage.
There is an advantage that -state can be turned on.

[0005] Further, there is used a type in which voltages having different signs are applied in a form in which an apparently uniform electrode is divided into four parts by developing an inverted Δβ directional coupler.

[0006] By dividing the electrode into multiple portions and applying voltages having different signs, the controllability of bar-state: on and cross-state: on is expanded. Therefore, the driving voltage can be reduced at the same time. Therefore, in addition to the above advantages, among the directional couplers described above,
An inverted Δβ directional coupler or a development of the same is preferably used.

[0007]

In a reversal Δβ directional coupler, etc., since a plurality of electrodes are formed, a temperature drift occurs based on a deviation in the size and form of these electrodes, and as a result, The drive voltage may not be stable. Therefore, as a measure against drift, SiO2
It is conceivable to provide a buffer layer of Si or the like in addition to the above. However, providing a thick buffer layer has a problem in that the driving voltage increases, and a problem in the Si layer is that drift cannot be completely suppressed. further,
Also for switching, it is necessary to divide the signal electrode, which makes it difficult to manufacture a traveling-wave-type electrode structure.

According to the present invention, the driving voltage is low, and several tens GHz.
Bar-state and cross-s at ultra-high speed
An object of the present invention is to provide a new directional coupler capable of completely turning on the state (100%) and further eliminating temperature drift in principle.

[0009]

According to the present invention, there is provided a substrate having an electro-optic effect, an optical waveguide formed in the substrate substantially parallel to a main surface of the substrate, for guiding a light wave, A directional coupler formed so as to cover at least a part of the optical waveguide, and an electrode for modulating the light wave, wherein the substrate has a plurality of polarization inversions in a traveling direction of the light wave. A directional coupler is characterized by being divided into domains.

FIG. 1 is a perspective view showing an example of the directional coupler of the present invention. A directional coupler 1 shown in FIG. 1 includes a substrate 2 having an electro-optical effect such as a lithium niobate single crystal.
And directional coupling type optical waveguides 3-1 and 3-2 made of titanium or the like formed in the substrate 2, a buffer layer 8 formed so as to cover these optical waveguides, and gold or the like. And electrodes 4-1 and 4-2.

The substrate 2 has a first domain-inverted domain having a polarization direction P substantially perpendicular to the main surface 2A of the substrate 2, and a second domain-inverted domain having a polarization direction Q opposite to the polarization direction P. And is divided into: That is, the substrate 1
It is composed of a pair of a first domain-inverted domain 5-1 and a second domain-inverted domain 5-2, which are two sets of domains that are adjacent to each other and have opposite polarization directions.

Further, the electrodes 4-1 and 4-2 are provided with a first domain-inverted domain 5-1 and a second domain-inverted domain 5-1, respectively.
The surface is divided into two equal parts by a boundary surface 7 with -2. Also,
The optical waveguides 3-1 and 3-2 are close to each other over a width L at a central portion of the optical waveguides, and light waves guided in the optical waveguides 3-1 and 3-2 are mode-coupled. . That is, in the directional coupler 1, the width L corresponds to the electrode length.

FIG. 2 is a diagram showing a switching state of light waves in the directional coupler of the present invention. Optical waveguide 3-1
Although the power of the light wave 6-1 incident on the optical waveguide 6-1 is initially “1”, when the optical waveguide 3-1 reaches the boundary surface 7 between the domain-inverted domains 5-1 and 5-2, Optical waveguide 3-2 by mode coupling having propagation constant difference Δβ
Between the electrodes 4-1 and 4-2 so that the power transfer rate to the

(Equation 1) A predetermined voltage is applied.

After the boundary 7, the domain-inverted domain 5-
In 2, the polarization wave is inverted with respect to the polarization inversion domain 5-1.

(Equation 2) Is equivalent to the voltage applied. Accordingly, since only the sign and the mode are mode-coupled with the opposite propagation constant difference-[Delta] [beta], the optical power moves to the optical waveguide 3-2 by following the reverse process and becomes "1". Therefore,
size

(Equation 3) Is applied, the cross-state is applied to the lightwave 6-1 incident on the optical waveguide 3-1.
Are completely (100%) turned on.

On the other hand, in order to turn on the bar-state, when the light wave 6-1 incident on the optical waveguide 3-1 reaches the boundary surface 7, the power is entirely changed to the optical waveguide 3-1.
To return to the size

(Equation 4) A predetermined voltage is applied. Thus, even in the case of passing the boundary surface 7 and reaching the domain-inverted domain 5-2, b
The ar-state ON state can be achieved.
Therefore, the size

(Equation 5) Is applied, the bar-state is completely (100%) turned on for the lightwave 6-1 incident on the optical waveguide 3-1.

The light wave 6-2 incident on the optical waveguide 3-2.
Ctos-state and b by the same process
The ar-state can be turned on.

FIG. 8 shows a switching diagram of the directional coupler type optical switch of the present invention. As is clear from this figure, by changing the electrode length and the signal voltage, ba is changed.
The r-state and the cross-state can be completely (100%) turned on, and the drive voltage (switching voltage:

(Equation 6) Voltage difference) can be reduced. Furthermore, as compared with the split electrode type inverted Δβ directional coupler, the directional coupler of the present invention can use a traveling-wave type electrode that is a uniform electrode, and can increase the number of divisions, so that high speed and It is characterized by being able to be driven at a low voltage and eliminating temperature drift.

In the present invention, the substrate 2 is divided into a plurality of domain-inverted domains 5-1 and 5-2 in the traveling direction of the light wave. Then, the interface 7 of these domain-inverted domains
Divides the electrodes 4-1 and 4-2 into two equal parts. Therefore, the electrodes 4-1 and 4-2 are substantially divided by the boundary surface 7, and are equivalent to those composed of two electrodes juxtaposed on the optical waveguide. As a result, the electrode configuration of the directional coupler of the present invention has the same electrode configuration as that of the inverted Δβ directional coupler, and the voltage can be reduced because it is easy to perform multiple division. In the configuration of the present invention, the boundary surface 7
, The polarization Ps of 5-1 and 5-2 is reversed,
Since the electrode areas 1 and 4-2 are each equal in size at the boundary surface 7, charges of equal and opposite directions are generated in the respective electrodes, but are canceled and no charge is generated. Therefore, 4-1 and 4-1
In principle, no drift which occurs as a source of temperature drift occurs between the electrodes.

On the other hand, in the directional coupler according to the present invention, the electrodes for actually modulating the light wave have uniform electrodes on the optical waveguides 3-1 and 3-2, similarly to the uniform Δβ directional coupler. Only two electrodes 4-1 and 4-2. Therefore,
The structure of the electrodes is simpler than that of an inverted Δβ directional coupler having four electrodes including two electrodes which are juxtaposed on each optical waveguide and apparently divided into two, and the formation of a traveling wave type electrode Is possible. As a result, ultra-high-speed switching becomes possible. Further, since the polarization direction of the substrate immediately below the electrode is opposite, the substrate is composed of a set of domains having the same area and adjacent to each other, so that the charges generated by the pyroelectricity cancel each other. This effect can suppress the temperature drift even in the case where the buffer layer is provided. Therefore, the occurrence of temperature drift and the like can be suppressed, and fluctuations in the drive voltage can be prevented.

That is, the directional coupler of the present invention can completely (100%) turn on the bar-state and the cross-state by changing the applied voltage, and can reduce the driving voltage. it can. Furthermore, compared to the split electrode type inverted Δβ directional coupler, the electrode configuration is simpler, and it is possible to avoid undesired phenomena such as temperature drift and easily achieve high speed. I have.

[0021]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail based on embodiments of the present invention. FIG. 1 is a perspective view showing an example of the directional coupler of the present invention as described above. In the directional coupler of the present invention, the substrate having the electro-optic effect needs to be divided into a plurality of domain-inverted domains in the traveling direction of the light wave. In the directional coupler 1 shown in FIG. 1, the substrate 2 has domains 5-1 and 5-
It is divided into two. However, the number is not limited as long as it is a set of domains adjacent to each other with opposite polarization directions in the substrate. In particular, when divided into four, the driving voltage can be reduced to 22% compared to the uniform Δβ type, or to 50% compared to the inverted Δβ type.

Number of divisions by polarization domain of electrode Sect
ion: N, electrode length: L (N), and drive voltage: V
FIG. 7 shows the result of simulating the relationship with (N). FIG. 7 is a plot of the minimum drive voltage Vo (N) and the electrode length L obtained for each division number N, as shown in FIGS. 8 and 10 below. Here, Lo and Vo (1) indicate the minimum complete coupling length Lo and the driving voltage when no voltage is applied in the conventional “directional coupler having a uniform electrode configuration without dividing the electrodes”. from this result,
When the number of divisions is increased, the drive voltage decreases, but the electrode length increases. Therefore, the upper limit of the polarization is preferably 30 divisions, and more preferably 10 divisions, because the electrode length becomes large and a large number of switches cannot be practically integrated.

The domain having the opposite polarization direction is:
It is preferable that the domain be composed of a pair of domains adjacent to each other and having opposite polarization directions, such as the positive and negative domains 5-1 and 5-2 shown in FIG. Accordingly, when the areas of the positive and negative domains immediately below the electrodes are equal when viewed as a whole substrate, the pyroelectric action is canceled and the temperature drift can be suppressed.

Therefore, it is necessary that an electrode for modulating the light wave is equally divided by at least a part of a boundary surface between the plurality of domain-inverted domains. FIG.
In the directional coupler 1 shown in FIG.
The substrate 2 is divided into two by a single boundary surface 7 of the domain-inverted domains 5-1 and 5-2. That is, in the case of a Z-cut plate, even division is desirable to prevent drift.
However, when using an X-cut plate and a Y-cut plate of a ferroelectric single crystal which is not affected by a pyroelectric effect, the number of boundary surfaces is not particularly limited. Therefore, when the number of polarization domains constituting the substrate is three or more, the domain can be equally divided at these two or more boundaries.

The number of equal divisions of the electrode at the boundary surface between a plurality of polarization domains is not particularly limited as long as it is divided into two or more as shown in FIG. However, the upper limit of the division is preferably 30 divisions, and more preferably 10 divisions, because it is larger than the minimum perfect coupling length Lo when no voltage is applied.
In practice, as shown in the calculation example in FIG. 7, the switching voltage can be sufficiently reduced even by dividing into two or four.

The domain-inverted domain 5-1 shown in FIG.
And 5-2 are polarized substantially perpendicular to the main surface 2A of the substrate 2. However, it is not an essential requirement that the polarization direction of the domain-inverted domain be substantially perpendicular to the substrate. If the polarization directions of the adjacent domains are opposite to each other, the domains may be polarized in a direction substantially horizontal to the main surface of the substrate. Further, the substrate may be polarized with a certain inclination angle with respect to the main surface of the substrate.

The material that can be used as the substrate of the present invention is not particularly limited as long as it has an electro-optical effect. However, lithium niobate single crystal, lithium tantalate single crystal, lithium niobate-lithium tantalate solid solution single crystal, potassium lithium niobate single crystal, potassium lithium niobate-potassium lithium lithium tantalate solid solution single crystal, and potassium titanyl phosphate single crystal It is preferable to use at least one kind of crystal.
These materials have a large electro-optic effect,
Excellent workability. Therefore, processing on a plate-like substrate is facilitated and formation of a domain-inverted domain is facilitated.

Further, the cut surface of these materials has an X-cut surface and a Y-cut surface depending on the polarization direction of the domain-inverted domain.
Any of a cut surface, a Z-cut surface, and an off-cut surface can be used.

The method for forming the domain-inverted domains is not particularly limited, and any method can be used.
However, as shown in FIG. 1, when a Z-cut surface of the above material is used so as to have a polarization direction substantially perpendicular to the main surface 2A of the substrate 2, the following is performed. That is, as shown in FIG. 1, in a case where the substrate 2 is composed of two domain-inverted domains 5-1 and 5-2, as shown in FIG. The electrode 12 is formed, and the uniform electrode 13 is formed on the main surface 11B opposite to the main surface 11A. When a predetermined voltage VA is applied between the electrodes at a predetermined rising speed, the polarization direction of the portion to which the voltage is applied is reversed. As a result, the domain-inverted domains 14- and 14 whose polarization directions are opposite to each other are obtained.
-2 is formed.

On the other hand, when three or more domain-inverted domains are formed, as shown in FIG.
A comb-shaped or watermark-shaped electrode 15 is formed on 1A in place of the plate-shaped electrode 12. When a predetermined voltage VB is applied between the comb-shaped or watermarked electrode 15 and the uniform electrode 13 at a predetermined rising speed, the polarization direction of the voltage-applied portion is reversed, and a plurality of polarizations are applied. Inversion domains 16 can be formed.

On the other hand, using the X-cut surface of the above material,
When forming a domain-inverted structure having a polarization direction substantially parallel to the main surface of the substrate, as shown in FIGS. 5 and 6, a plate-shaped electrode 22 and a rod-shaped electrode 23 or a comb-shaped electrode are formed on the same main surface 21A of the substrate 21. The electrode 25 and the bar-shaped electrode 23 are formed. Next, when a predetermined voltage VC or VD is applied between these electrodes, the two domain-inverted domains 24-1 and 24-2
-2 or a plurality of domain-inverted domains 26 are formed.

The electrodes 12, 13, and 15 can be formed from tantalum, gold, aluminum, platinum, chromium, copper, or the like.

[0033]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be specifically described based on embodiments. Embodiment 1 In this embodiment, a directional coupler 1 as shown in FIG.
Was formed. (Production of Directional Coupler) As the substrate 2, a thickness of 0.5 mm
3 inch Z-cut lithium niobate single crystal wafer was used. A plate electrode 12 as shown in FIG. 3 was formed on one main surface 2A of the wafer by photolithography, and a uniform electrode 13 was formed on the other main surface 2B. Then, a voltage of 20 kV / mm and a rise time of 0.5 msec are applied between these electrodes, and two domain-inverted domains 5-1 equally divide the substrate in the length R direction as shown in FIG. And 5-2 were formed.

Next, the substrate 2 is formed by photolithography.
After patterning on the main surface 2A, proton exchange was performed by immersion in benzoic acid at 200 ° C. for 30 minutes. Next, an annealing treatment is performed at 300 ° C. for 4 hours.
Optical waveguides 3-1 and 3-2 were formed. At this time, the difference Δ between the effective refractive index of the waveguide part and the substrate was 4 × 10 ⁻³ , and the distance between the waveguides was 4 μm. Next, SiO ₂ is formed to a thickness of 0.2 μm as a buffer layer, and traveling-wave type electrodes 4-1 and 4-2 made of gold are formed over the coupling portion L by a vapor deposition method, and the length R Was cut into chips of 50 mm in width and 5 mm in width. The electrode L at this time is 25 mm
Met. Thereafter, both end faces of the chip were polished to form a directional coupler 1 as shown in FIG.

(Evaluation of Characteristics of Directional Coupler: Evaluation of Temperature Drift) An optical fiber is coupled to the optical waveguides 3-1 and 3-2 of the directional coupler 1 obtained as described above, and a wavelength of 1550 is obtained.
The temperature drift was evaluated by introducing a light wave of nm.
As a result, since the areas of the electrodes 4-1 and 4-2 are equally divided in the domain opposite to the polarization direction, the temperature drift is not observed because the pyroelectric charges induced by the temperature change cancel each other out. .

Further, a light wave having a wavelength of 1550 nm is introduced to simulate the difference between the applied voltages (driving voltage) at which the bar-state and cross-state are turned on (drive voltage) and the electrode length L based on the following equations (1) and (2). FIG. 8 shows that the driving voltage was minimum when L / Lo was 2.7 (FIG. 9). As described above, the electrode length 25
When a directional coupler of mm was manufactured, the driving voltage was 2.9.
V, and the maximum modulation speed was 10 GHz.

(Equation 7) L: electrode length Lo: minimum perfect coupling length in uniform electrode Δβ type directional coupler k: coupling coefficient Δβ: propagation constant difference

Embodiment 2 After forming a domain-inverted domain, an optical waveguide, and a traveling-wave electrode in the same manner as in Embodiment 1, a KrF excimer laser was used.
The surface opposite to the substrate surface on which the waveguide and the traveling wave electrode were formed was processed by a spot scan method. The size of the irradiated spot is 1.0 mm in the operation direction and 0.2 m in width.
m, and the irradiation energy was 6 J / cm ² . The pulse width was 15 nsec, the pulse frequency was 600 Hz, and the operation speed was 0.1 mm / sec. As a result, the maximum modulation speed was widened to 40 GHz. Also, no temperature drift was observed.

Embodiment 3 (Production of Directional Coupler) In this embodiment, the substrate is divided into four equal parts, and the domain-inverted domains 5-1 and 5- as shown in FIG.
2 are adjacent to each other, and three boundary surfaces dividing these four domains form a directional coupler in which the electrode is divided into four equal parts. The directional coupler was manufactured in the same manner as in Example 1 except that the comb electrode 15 shown in FIG. 4 was used instead of the plate electrode 12 shown in FIG. In addition,
The electrode length in this case was 41 mm.

Simulation of the electrode length L and the driving voltage based on the following equations (3) and (4) was performed in the same manner as in Example 1, and the results are as shown in FIG. 10, where L / Lo is 4.5. The driving voltage became minimum (FIG. 11). When a directional coupler having an electrode length of 41 mm was actually manufactured, the driving voltage was 1.6 V and the maximum modulation speed was 10 GHz. Furthermore, no temperature drift was observed as in Example 1.

(Equation 8) L: electrode length Lo: minimum perfect coupling length in uniform electrode Δβ type directional coupler k: coupling coefficient Δβ: propagation constant difference

Example 4 (Production of Directional Coupler) In this example, domain-inverted domains having a polarization direction substantially parallel to the main surface of the substrate were formed in the substrate. In manufacturing an actual directional coupler, an X-cut lithium niobate single crystal having the same size as that of Example 1 was used. Next, a plate-like electrode 22 and a rod-like electrode 23 as shown in FIG. 5 were formed on the substrate by photolithography. Then, a start-up time of 0.1 mm between these electrodes.
A voltage of 20 kV / mm was applied for 5 msec to divide the substrate equally in the length R direction to form two domain-inverted domains having a polarization direction substantially parallel to the main surface of the substrate. Next, an optical waveguide and a traveling-wave-type electrode were formed in the same manner as in Example 1 to obtain a final directional coupler. The electrode length in this case was 30 mm as in Example 1. No temperature drift was observed.

(Evaluation of Characteristics of Directional Coupler) After connecting an optical fiber in the same manner as in Example 1, the drive voltage and the maximum modulation speed were examined. As a result, the driving voltage was 4 V, and the maximum modulation speed was 10 GHz.

Embodiment 5 In this embodiment, a directional coupler type optical waveguide is first formed by Ti diffusion, and then two domain-inverted domains equally divided in the length R direction of the substrate and A corrugated electrode was formed to produce a directional coupler type as shown in FIG. At this time, the difference Δn in the effective refractive index between the waveguide portion and the substrate was 3 × 10 ⁻³ , and the interval between the waveguides was 4 μm. As a result, the electrode length was reduced to 16 mm as compared with Example 1,
On the contrary, the drive voltage increased to 4.4V. This is because T
The mode coupling between the waveguides 6-1 and 6-2 is increased due to the small refractive index difference Δn of the i-diffused waveguide, the electrode length is reduced, and conversely, the mode size of the waveguide is increased.
It is considered that the drive voltage increased due to the decrease in the directional electric field. Also, no temperature drift was observed as in Example 1.

Example 6 A Zn-doped LPE film was formed on a z-cut lithium niobate substrate in order to increase the difference Δn between the effective refractive index of the waveguide portion and the substrate, and then the directional coupler was etched. An optical waveguide was formed. Next, two domain-inverted domains and a traveling-wave electrode were equally formed in the direction of the substrate length R in the same manner as in Example 1, and a directional coupler-type optical switch as shown in FIG. 1 was manufactured. At this time, the difference Δn between the effective refractive index of the waveguide portion and the substrate was 5 × 10 ^{−3, and the} waveguide interval was 4 μm. As a result, as compared with Example 1, the electrode length was increased to 36 mm, and the driving voltage was reduced to 1.8 V on the contrary. No temperature drift was observed as in the example.

Comparative Example 1 (Preparation of a directional coupler) The same procedure as in Example 1 was repeated except that Z-cut lithium niobate was used as a substrate material and no domain-inverted domain was formed in the substrate. Similarly, a directional coupler was manufactured.

(Evaluation of Characteristics of Directional Coupler) When the drive voltage and the maximum modulation speed were examined in the same manner as in Example 1, temperature drift was observed. The driving voltage is 7.3V
And the maximum modulation speed was 100 MHz.

Comparative Example 2 As a substrate 2, a proton-exchanged optical waveguide was formed in the same manner as in Example 1 except that z-cut lithium niobate having the same size as in Example 1 was used. Then, S
An iO ₂ buffer layer and an inverted Δβ type electrode pattern were formed, and an electrode made of Au was formed by an evaporation method. As a result, the driving voltage was 5 V, and the maximum switching speed was 1 GHz.

As is clear from Examples 1 to 3,
The directional coupler of the present invention has a bar
It can be seen that -state and cross-state are turned on, that is, switching is possible. Further, from Examples 1 to 6 and Comparative Examples 1 and 2, the directional coupler of the present invention has a lower driving voltage than a uniform Δβ or inverted Δβ directional coupler having no domain-inverted domain in the substrate. It can be understood that high-speed modulation can be performed without temperature drift.

As described above, the present invention has been described in detail based on the embodiments of the present invention with specific examples, but the present invention is not limited to the above contents and does not depart from the scope of the present invention. In the above, all changes and modifications are possible.

[0049]

As described above, according to the directional coupler of the present invention, there is no occurrence of temperature drift or the like, the driving voltage is low, and the bar-state and cross-state states are controlled only by controlling the applied voltage. That is, switching becomes possible.

[Brief description of the drawings]

FIG. 1 is a perspective view showing an example of a directional coupler of the present invention.

FIG. 2 is a diagram showing a switching state of a light wave in the directional coupler of the present invention.

FIG. 3 is a perspective view illustrating an example of a method of forming a domain-inverted domain.

FIG. 4 is a perspective view showing another example of a method for forming a domain-inverted domain.

FIG. 5 is a perspective view showing still another example of a method of forming a domain-inverted domain.

FIG. 6 is a perspective view showing another example of the method of forming the domain-inverted domains.

FIG. 7 is a graph showing simulation results of a minimum drive voltage and an electrode length at each division number of an electrode.

FIG. 8 is a graph showing an example of a simulation result of a drive voltage and an electrode length.

FIG. 9 is a graph showing a relationship between a normalized switching voltage and a normalized interaction length.

FIG. 10 is a graph showing another example of a simulation result of a driving voltage and an electrode length.

FIG. 11 is another graph showing the relationship between the normalized switching voltage and the normalized interaction length.

[Explanation of symbols]

1 directional coupler, 2, 11, 21 substrates, 3-1, 3
-1 optical waveguide, 4-1 and 4-2 electrodes, 5-1 and 5-
2, 14-1, 14-2, 24-1, 24-1 domain-inverted domains, 6-1 and 6-1 light waves incident on an optical waveguide,
7 boundary surface, 8 buffer layer, 12,22 plate electrode,
13 Uniform electrodes, 15, 25 Comb electrodes, 16, 26
Multiple poled domains, 23 rod-shaped electrodes

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Tatsuo Kawaguchi 2-56 Suda-cho, Mizuho-ku, Nagoya-shi, Aichi Japan Inside Nihon Insulators Co., Ltd. No. 56 F-term in Nihon Insulators Co., Ltd. (Reference) 2H079 AA02 AA12 BA01 CA05 DA03 DA18 EA04 EB04 HA12 HA23 2K002 AB04 BA06 CA02 CA03 CA13 DA07 EA04 FA27 HA03

Claims (4)

[Claims] 1. A substrate having an electro-optic effect, an optical waveguide formed in the substrate substantially parallel to a main surface of the substrate, for guiding a light wave, and an electrode for modulating the light wave. Wherein the substrate is divided into a plurality of domain-inverted domains in the traveling direction of the light wave, and the electrode is at least a part of a boundary surface of the domain-inverted domains. A directional coupler, which is equally divided by 2. The directional coupler according to claim 1, wherein a polarization direction of the domain-inverted domain is substantially perpendicular to a main surface of the substrate. 3. The directional coupler according to claim 1, wherein the plurality of domain-inverted domains comprise a set of even-numbered domains adjacent to each other and having opposite polarization directions. 4. The substrate comprises a lithium niobate single crystal, a lithium tantalate single crystal, a lithium niobate-lithium tantalate solid solution single crystal, a potassium lithium niobate single crystal, a potassium lithium niobate-lithium potassium tantalate solid solution single crystal. The directional coupler according to claim 1, comprising at least one of a crystal, a potassium titanyl phosphate single crystal, and a gallium-arsenic single crystal.